Method and apparatus for phasing a linear accelerator



United States Patent METHOD AND APPARATUS FOR PHASING A LINEAR ACCELERATQR David J. Goerz and Richard B. Neal, Menlo Park, and Kenneth B. Mallory, Palo Alto, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Apr. 27, 1960, Ser. No. 25,160 6 Claims. (Cl. SIS-5.42)

The present invention relates generally to a method and apparatus for phasing a linear accelerator. In particular, the invention relates to a linear accelerator which uses a traveling electromagnetic wave to accelerate a beam of electrons wherein the phasing operation is accomplished by using phase comparison techniques to optimize the phase relationship between the beam and the traveling wave.

In general, a traveling wave linear electron accelerator is composed of a plurality of separate loaded waveguide sections disposed end to end along the longitudinal axis of the accelerator. Each section is coupled to a source of microwave power whereby a traveling electromagnetic wave is created in each section. The accelerator structure is designed to have the phase velocity of this wave travel at about the velocity of light so that a beam of electrons injected into the accelerator can be made to ride along with the traveling wave. In this manner a very high energy electron beam can be created. Such high energy beams have widespread utility in the field of nuclear research as well as an increasing use for internal medical treatment on human beings. As the theory and design of linear accelerators are well known to those skilled in the art, little mention is made hereinafter regarding the details of construction or principles of operation of a linear accelerator.

A beam of charged particles moving with a traveling electromagnetic wave receives maximum power from the wave when the bunches of electrons which comprise the beam are positioned coincident in space with the crest of the traveling wave. As the electron beam travels through the successive sections it receives energy from the traveling wave of each section. Now, in order for the beam to obtain a maximum energy upon reaching the output end of the accelerator the beam must be in optimum phase relationship with the traveling Wave of each individual section.

More specifically, the power input to each section must be phased to create a peak magnitude at the point where an electron bunch enters into the section at the particular instant when the bunch enters into the section. If these conditions are met then the beam travels through the section positioned at the crest of the traveling wave. When the phase of the input power is incorrect the electron bunch does not ride the wave crest and hence receives less than maximum energy. Therefore, the problem at hand is to couple radiofrequency power into all the accelerator sections with the proper phase relationships to achieve maximum energy transfer to the electron beam.

Present methods being used to phase existing linear traveling wave accelerators comprise, in general, varying the phase of each of the separate power inputs while observing the output energy of the electron beam. More particularly, the electron beam is turned on and microwave power is coupled into the first accelerator section only. The phase of the input to the first section is ad justed until a maximum beam energy is observed at the terminal end of the accelerator. It may be noted that beam energy is generally measured by magnetic deflection techniques. The second section is then energized and the same adjusting procedure is repeated. This process is continued until each section is tuned thereby giving a substantially maximum electron beam energy. It should be noted that throughout the specification the term first section referred to is meant to be the first section which must be phased, i.e., if a buncher is not used with the electron gun, the first section in line after the gun acts as a buncher and it is actually the second section in line which is the first section to be phased.

The relatively simple conventional phasing methods of the foregoing type have been found to be adequate for existing accelerators. However, the problem of phasing a very long accelerator, for example, one two miles long having 1000 individual power inputs, is more complicated than that of phasing a relatively short accelerator, e.g., the 220 foot long Stanford Mark III accelerator having only twenty-one power inputs described in the Review of Scientific Instruments, vol. 26, No. 2, February 1955, pp. 134204.

The conventional phasing method of maximizing the beam energy by separately adjusting each power input is not adaptable to a very long accelerator because of two prime deficiencies. First, when the initial sections are being adjusted, the electron beam does not receive enough energy to be transmitted to the end of the accelerator. This shortcoming could conceivably be solved by trial and error means of just getting a beam started through the accelerator and maximizing the energy afterwards. Such a scheme, however, is very time consuming and especially diflicult with a long accelerator. The most serious drawback, however, of the conventional method is caused by the large number of power inputs necessary for a very long accelerator. It is extremely difficult to perceive any variation of beam energy at the accelerator output while varying the phase of one power input when a great many other sections are also energized. Because in this situation beam energy variations are extremely small, the conventionad method has a very low sensitivity when applied to a long accelerator and hence the phasing operation would be a tedious and time consuming job.

The present invention is a particularly useful scheme for phasing very long accelerators, but nevertheless its nature is such that it can be readily used with advantages to phase an accelerator of any length. Rather than employ beam energy observations, the invention utilizes the phase relation of the electromagnetic energy at different points in each accelerator section. Whereas minute beam energy variations are difiicult to measure, there do exist phase comparison means which can quite accurately determine the relative phase of wave energy throughout the accelerator.

Briefly, the principle of the invention is to observe a phase shift of the resultant traveling wave in an accelerator section when the electron beam is turned on, as compared to the traveling wave in the section with the beam off. This phase shift occurs only when the electron beam is not optimally phased with the traveling wave, hence the degree of phase shift is used as a measure of the degree to which the beam and traveling wave are out of phase.

In addition to its particular usefulness with a long accelerator, the present invention has the advantage of exhibiting a higher sensitivity than conventional methods used with relatively short accelerators. This allows the phasing operation to be performed to within virtually any degree of accuracy which may be practically required.

One very important improvement of the invention over conventional phasing schemes is the short time in which the phasing operation may be completed. For example, when using a conventional phasing method it takes about eight hours to properly tune the Stanford Mark III accelerator which is 220 feet long. Using the present invention it would take only 25 minutes to correctly phase an accelerator which is two miles long.

Another advantage of the present invention is that it does not detract from the useful length of the accelerator. Rather than insert beam energy measuring means between accelerator sections it is necessary only to insert a phase shifting and detecting circuit between the input and output coupling circuits of each section externally thereto.

Accordingly, it is the main object of the present invention to provide a method and apparatus for optimumly phasing the power inputs to the sections of a linear traveling wave accelerator.

Another object of the invention is to provide a phasing method and apparatus which are particularly suitable for very long linear accelerators.

It is a further object of the present invention to provide a phasing means and method having a high sensitivity characteristic affording extremely accurate maximizing of the electron beam energy.

A still further object is to provide a phasing apparatus and method which does not detract from the useful length of a linear accelerator.

An even further object is to provide a method and apparatus which very quickly completes the operation of phasing a linear accelerator.

The invention, both as to its apparatus and method, together with further objects and advantages thereof will be better understood by reference to the following specification taken in conjunction with the accompanying drawing, of which:

FIGURE 1 is a schematic diagram depicting generally an arrangement of apparatus as employed in a multisection traveling wave accelerator to phase same in accordance with the present invention;

FIGURE 2 depicts various vector representations of the electromagnetic fields present in a linear accelerator at various phase relationships relative to each other, portion (a) being indicative of an optimum phase relation between the electron bunch and traveling wave, and portion (b) being indicative of an out of phase relationship between the bunch and wave; and

FIGURE 3 is a schematic diagram of a preferred embodiment of the apparatus of the invention having particular application with a very long accelerator.

Referring now to the drawings, there is shown a traveling wave linear accelerator 11 of generally conventional construction. Accelerator 11 comprises a plurality of loaded waveguide structures referred to as accelerator sections, several of such sections being depicted as 12 and 12. The plurality of accelerator sections are disposed coaxially end to end along the longitudinal axis of accelerator 11. An electron gun 13 is coaxially mounted to section 12 at the front end of accelerator 11. A power supply 14 activates gun 13 through a switch 16. Electron gun 13 injects into the accelerator a pulsed beam of electrons designated by dashed lines 17. The electron beam 17 travels from gun 13 through section 12 into section 12' and so on down through the plurality of sections to the target end of the accelerator.

An oscillator 18 such as a klystron is provided in the usual manner to supply the radiofrequency power for driving the accelerator. Oscillator 18 couples into a distribution waveguide 19 which extends the length of the accelerator. A radiofrequency amplifier 21, e.g., a pulsed klystron, couples power out from waveguide 19 through a phase shifter 22. The microwave power output of amplifier 21 is in turn coupled through a directional coupler 23 into the first section 12 at a portion thereof nearest the electron gun 13. The output end of section 12 is coupled to a load 24.

In accordance with the present invention, a portion of the microwave power from amplifier 21 is diverted into a phase shifter 26, and a phase detector 27 is coupled between phase shifter 26 and the output end of the section 12 whereby a phase comparison can be made between the power extracted from the terminal end of section 12 and the power coupled into the initial portion of section 12. To facilitate diversion of the power in this manner, directional coupler 23 is employed in the connection of amplifier 21 to the input end of the section, one arm of the coupler feeding the phase shifter 26. Similarly a directional coupler 28 connects the output end of the section to load 24 whereby coupling to the phase detector 27 is simultaneously facilitated through another arm of the coupler. Both couplers 23 and 28 are terminated in their remaining arms respectively by matched loads 29 and 31.

A similar arrangement of components to those coupled into section 12 is coupled into section 12 and the like components are designated by primed numerals. Similar arrangements are also coupled into each of the sections which comprise the length of the accelerator.

As regards the individual components of the arrangement, it should be noted that the phase detector 27 must be independent of the power level of signals fed into its inputs. This requirement is apparent upon considering the difference of wave energy between the front and terminal portions of an accelerator section. In addition, the detector should be of the type that provides a null reading when the inputs from shifter 26 and coupler 28 have exactly the same phase. When the two inputs to detector 27 are not in phase, it is preferable that the degree to which these inputs are out of phase be directly indicated as by means of a meter.

The phase shifter 26 as well as the input power phase shifter 22 may, for example, be of the slotted waveguide type illustrated in Microwave Transmission Circuits, Ragan, vol. 9, Radiation Laboratories Series, McGraw- Hill, 1948, p. 513, or they may be of the dielectric type illustrated in the same work at p. 514. In practice, a phase shifter of the dielectric type has been proven most advantageous.

Considering now the method of the invention as facilitated with the arrangement of components described hereinbefore to phase the sections of the accelerator, microwave power from the oscillator 18 is first coupled into the distribution waveguide 19. The amplifier 21 is then pulsed on to introduce a traveling electromagnetic wave into the first section 12. The phase of the energy coupled into the front portion of the section, viz., the starting phase of the wave, is compared to the phase of the wave at the output end of the section and this is accomplished by observing the indication of phase departure at phase detector 27. Such phase relationship is indicated at the detector inasmuch as the coupler 23 diverts a portion of the input energy through phase shifter 26 to one input of the detector whereas the coupler 28 diverts a portion of the output load energy to the other input of the detector.

The phase angle indicated at the detector is the phase shift of the unloaded wave in propagating through the section 12 and is employed herein as a reference phase angle. Although the reference phase angle per so may be utilized for subsequent comparison purposes, it is more convenient that departures from the reference phase angle be indicated at the phase detector as departures from zero. Accordingly, as the next step of the method a null reading is established at the phase detector by adjusting the phase shifter 26 compensatory to the reference phase angle. In other words, the reference phase angle between the unloaded wave at input and output of the section is established as a null reading of the phase detector.

After the reference phase angle is established, the electron gun 13 is turned on by switch 16 and a beam of electron bunches is injected axially into section 12 and transmitted therethrough. The electromagnetic field in section 12 now has two components. One is that of the driving electromagnetic wave introduced from the radiofrequency amplifier 21 and the other is that caused by the presence of the electron beam designated by line 17. These two electromagnetic waves co-act to create a resultant loaded traveling wave in the section.

When the electron bunch is positioned in space exactly coincident with the negative peak of the driving wave, the phase of the resultant loaded wave remains the same as that of the unloaded Wave, viz., the wave before the beam was turned on. However, when the electron beam is not positioned exactly coincident with the crest of the driving wave, a shift in the phase of the resultant loaded wave takes place throughout the length of the section. The foregoing will be more fully understood upon reference to the vector diagrams of FIGURE 2. FIGURE 2a vectorially represents the orientation of fields in an accelerator section when the electron bunch is in phase with the driving electromagnetic wave. Vector 32 represents the wave induced by the presence of the electron beam and the driving wave in the section is shown by vector 33. Vector 34 represents the resultant wave created by adding the vectors 32 and 33. It is seen that the resultant wave retains exactly the same phase as the driving wave.

Now consider the situation which exists when the electron bunch is out of phase with the driving wave by an angle 6, as shown in FIGURE 2b. Vector 32' represents the electromagnetic field induced by the electron bunch, vector 33' represents the field caused by the driving traveling Wave, and vector 34' represents the field of the resultant traveling wave. It is seen that for this outof-phase situation the phase of the resultant traveling wave is shifted from that of the unloaded driving wave by an angle From the foregoing description, it is seen that the angle phase shift between the loaded wave and unloaded or reference wave is a measure of departure from the optimum phase relation between the electron beam bunches and driving electromagnetic wave wherein the beam bunches are in phase with the negative peaks of the wave. In accordance with the latter steps of the method of the invention, the phase shift 5 is determined and the phase of the driving energy applied to the input end of the section Varied to compensate the shift. When the shift is reduced to zero, viz., when =0, the electron bunches are in phase with the negative peaks of the driving wave and the section 12 is hence optimumly phased. The foregoing steps are accomplished merely by observing the phase detector 27 and adjusting phase shifter 22 until the detector indicates a null.

Successive sections of the accelerator are phased in similar fashion to that detailed hereinbefore relative to the first section 12. More specifically, to achieve phasing of the whole accelerator length the following procedure is advantageously followed. Before turning on the electron gun, a reference phase angle is established for each of the accelerator sections in the manner described hereinbefore. That is, power is coupled into each of the sections from their associated sources of driving energy including radiofrequency amplifiers 21 and a null reading on each of the phase detectors 27 is obtained by varying the respective phase shifters 26. After reference phase angles have been established, switch 16 is used to turn the electron gun on. Now, the phase shifter 22 is adjusted until the detector 27 exhibits a null reading indicating that section 12 is properly tuned. Shifter 22' is then adjusted to tune section 12. This procedure is repeated in succession down the accelerator until each section is correctly phased.

It is particularly important to note the extreme sensitivity attainable in phasing the sections of an accelerator in accordance with the invention. More specifically, the sensitivity of the phasing method is determined by the relationship, derived hereinafter, between the measured angle (phase shift between unloaded and loaded wave) and the phase angle 0 between the electron bunches and negative peaks of the wave. From the law of cosines, it is seen that E =E +E 2E E cos 0 where E and E are the magnitudes of the electromagnetic field components due to electron bunches and driving wave present in the accelerator section, and E is the resultant field. Now, from the law of sines,

sin sin 0 EG By substituting E from Equation 1 into Equation 2,

sin 0 3 EW 2 E 1/2 (s; E "1 Taking the differential of Equation 3 yields the relation,

A cos 0 A* sin 9 0 Considering now the limit of dq5/d0 as 0 approaches zero the following expression relating 5 and 0 obtained,

Equation 6 thus shows the variation of the measured angle 5 with respect to the actual phase angle, 0, between the electron bunches and driving wave. Hence Equation 6 is a measure of the sensitivity of the instant phasing method. Equation 6 can also be used to directly give the angle 6 when the angle is known.

To illustrate the extreme sensitivity of the method, the sensitivity factor of Equation 6 is hereinafter evaluated relative to the parameters of a section of an actual accelerator, namely, the Mark IV accelerator constructed at the Hansen Microwave Laboratory of Stanford University. The ratio E /E appearing in Equation 6 may be readily expanded into terms of the operating parameters of an accelerator from the equations:

From Equations 7 and 8 it can be shown that:

a m WE E, 2' Lr (e 1) For the Mark IV accelerator P 15 megawatts i=5 10 amps L=305 cm.

r=0.47 10 ohms/cm. IL=0.9 neper Upon substituting these parameters into Equation 9,

w -5.9 Therefore substituting this value into Equation 6, the sensitivity is seen to be:

From Equation 9 it is seen that for a given accelerator structure, the sensitivity of the instant method and apparatus increases with a decrease of the input power P or with an increase of the electron current i. This characteristic lends itself quite beneficially to the operation of phasing an accelerator. Note, for example, that if P;

in the above actual example was 1.5 megawatts instead of megawatts, then the sensitivity as given by Equation 6 would be 1.15 instead of 0.2. Consequently, during the phasing of the accelerator the driving power P can be reduced as much as is necessary to accurately complete the phasing operation. Then, after phasing is achieved, the driving power can be increased to whatever level is desired for the acceleration of the electron beam. For sake of comparison it is noted that a sensitivity between 0.1 and 0.2 is accepted as adequate for satisfactorily phasing an accelerator. Hence, as the invention is capable of a sensitivity more than five times better than that required for adequate phasing, it is seen that phasing can be achieved to within any practical requirement of accuracy.

It will be appreciated that in the case of a very long accelerator economic considerations prohibit the use of separate detector circuitry with each accelerator section. Hence an alternative embodiment of the phasing apparatus comprising a single detector circuit coupled through a switching circuit to a plurality of accelerator sections is preferably employed with a very long accelerator, and such embodiment is provided as illustrated in FIGURE 3. As shown therein, a linear accelerator 36 comprised of a plurality of substantially identical sections 37, 37', 37 etc., is provided. An electron gun 38 is mounted coaxially to section 37 and is actuated by a power supply 39 through a switch 41. An electron beam as designated by dashed lines 42 is injected axially into accelerator 36 by gun 38. A microwave oscillator 43 feeds into a distribution waveguide 44. Power is coupled out from waveguide 44 into a radiofrequency amplifier 46 through a phase shifter 47. The output from amplifier 46 is coupled into a front portion of section 37. Power is coupled from waveguide 44 into a front portion of sec tion 37 through a phase shifter 47' and radiofrequency amplifier 46'. Power is similarly coupled into the front portion of each identical section in a manner substantially the same as that for sections 37 and 37.

Power is coupled out of section 37 at a terminal portion thereof through a directional coupler 48 into a load 49. Coupler 48 diverts part of the output power through a transmission line 51 into a coaxial switch 52. A similar output coupling circuit to that of section 37 is also connected to a terminal portion of section 37 as is shown by load 49', directional coupler 48, and transmission line 51, with line 51 coupling into coaxial switch 52. The remaining arm of coupler 48 is terminated in a matched impedance 53 as is coupler 48 by impedance 53' and similarly for each other section. Switch 52 is designed to couple power from one of its transmission line inputs at a time into an input of a phase detector 54. Detector 54 has another input coupled thereto from oscillator 43 through a phase shifter 56.

In conducting the method of the invention with the instant embodiment, the phase of the output from oscillator 43 is designated as the reference phase. Section 37 is phased by first turning on amplifier 46 and coupling power out of section 37 into detector 54 through switch 52. Shifter 56 is then adjusted to indicate the reference phase angle between the reference phase and the output phase of section 37 as a null reading on detector 54. Electron gun 36 is now turned on by switch 41 and shifter 47 is adjusted to give a null reading on the detector. Section 37 is now correctly phased. The electron gun is turned off by switch 41 and switch 52 is changed to couple power from section 37' through line 51' into the detector. Amplifier 46 is turned on and shifter 56 is adjusted to give a null reading. Switch 41 is used to initiate the electron beam again and shifter 47 is adjusted to give a null reading whereby section 37 is now correctly phased. This procedure is repeated successively down through each of the sections which are connected to switch 52.

Consider now the instant embodiment as applied to a very long accelerator having, for example 1000 sections. A compromise between the excessive cost of a large number of detectors and the complexity of long transmission line circuitry dictates that each switch 52 and detector 54 can most advantageously handle about 25 accelerator sections. Such a group of sections is designated as a sector. This means that 40 detectors and 40 coaxial switches are used with the 40 sectors. For reasons other than phasing procedures, a very long accelerator can be used to best advantage by having an electron gun optionally mountable with each sector, e.g., in this manner various different lengths of accelerator can be used for different experiments and requirements. By using this plurality of electron guns, the 40 sectors can be individually phased at the same time. Then, in the manner of the invention, the whole accelerator length can be phased by using only one electron gun and considering each sector analogous to a single section. The foregoing procedure is exceptionally rapid and the job phasing a 1000 section accelerator can be completed in about 25 minutes.

While the present invention has been hereinbefore described in terms of specific steps in the method and with respect to but several embodiments, it will be apparent that numerous modifications and variations are possible within the spirit and scope of the invention and thus it is not intended to limit the invention except by the terms of the following claims.

What is claimed is:

1. In an apparatus for phasing a linear accelerator waveguide section, the combination comprising a variable phase microwave power source coupled into said section at the front portion thereof, phase varying means associated with said power source to provide a constant phase radiofrequency reference signal, a phase shifter, a phase detector serially connected at one end thereof to said phase shifter and at the other end thereof to the terminal portion of said section, said phase shifter adapted to receive said reference signal at the free end thereof.

2. In an apparatus for phasing a linear accelerator having at least one loaded waveguide section, the combination comprising a phase shifter, a phase detector, said shifter and detector connected serially between the front and terminal portions of said section, and a variable phase microwave power source coupled into each section at said front portion thereof.

3. In an apparatus for phasing a linear accelerator having at least one loaded waveguide section, the combination comprising means for introducing variable phase microwave power to said section, means for measuring the relative phase angle between the phase of a traveling electromagnetic wave in each loaded waveguide section of a linear accelerator at a front portion of each section and the phase of said wave at a terminal portion thereof, means for introducing an electron beam into each section, and means for varying the phase of the traveling wave in each section while the beam is passing therethrough to maintain the phase thereof equal to the measured relative phase angle.

4. In an apparatus for phasing a linear accelerator, the combination comprising at least one loaded waveguide section, a variable phase microwave power source coupled through a first directional coupler into a front portion of each section, a radiofrequency load coupled through a second directional coupler into a terminal portion of each section, and a phase detector coupled between the first and second directional couplers of each section.

5. In an apparatus for phasing a linear accelerator having at least one loaded waveguide section, the combination comprising a variable phase microwave power source coupled through a first directional coupler into a front portion of each waveguide section of a linear accelerator, a phase shifter connected to said first directional coupler, a null reading phase detector connected to said phase shifter, a second directional coupler connected to said detector, and a radiofrequency load coupled through said second directional coupler to a terminal portion of said section.

6; In an apparatus for phasing a linear accelerator having a plurality of loaded Waveguide sections, the combination comprising a plurality of radiofrequency amplifiers respectively coupled into the waveguide sections at the front portions thereof, a single microwave oscillator commonly connected to said amplifiers, a phase shifter connected to said oscillator, 21 phase detector connected to said phase shifter, a coaxial switch having a single input terminal connected into said detector and a plurality of selectable output terminals, and a plurality of transmission lines respectively connected between the terminal portions of said sections and the output terminals of said coaxial switch.

References Cited in the file of this patent UNITED STATES PATENTS 2,556,978 Pierce June 12, 1951 2,813,996 Chodorow Nov. 19, 1957 2,883,536 Salisbury et a1 Apr. 21, 1959 2,899,598 Ginzton Aug. 11, 1959 2,920,228 Ginzton Jan. 5, 1960 2,925,522 Kelliher Feb. 16, 1960 2,993,141 Post July 18, 1961 3,070,726 Mallory Dec. 25, 1962 

1. IN AN APPARATUS FOR PHASING A LINEAR ACCELERATOR WAVEGUIDE SECTION, THE COMBINATION COMPRISING A VARIABLE PHASE MICROWAVE POWER SOURCE COUPLED INTO SAID SECTION AT THE FRONT PORTION THEREOF, PHASE VARYING MEANS ASSOCIATED WITH SAID POWER SOURCE TO PROVIDE A CONSTANT PHASE RADIOFREQUENCY REFERENCE SIGNAL, A PHASE SHIFTER, A PHASE DETECTOR SERIALLY CONNECTED AT ONE END THEREOF TO SAID 